Is there a reason workflow builders in F# don't use interfaces?

This is a question just out of curiosity: when you implement a workflow factory, you don't do it as an interface implementation, but rather just make sure the function signatures of the monad functions match. Is there a design开发者_如何学编程 reason for this?

For one thing, the lack of higher-kinded types in .NET means that you can't give the methods useful signatures. For instance, ListBuilder.Return should have type 't -> 't list, while OptionBuilder.Return should have type 't -> 't option. There's no way to create an interface with a Return method that has a signature supporting both of these methods.

I think that the lack of higher-kinded types as mentioned by kvb is probably the main reason. There are ways to workaround that, but that makes the code a bit obscure (see this snippet).

Another reason is that F# computation expressions allow you to define different combinations of methods. It's not always just Bind and Return. For example:

• Some define Yield, YieldFrom, Combine to allow generating results
• Some define Return, ReturnFrom, Bind to define a monad
• Some define Return, ReturnFrom, Bind, Combine to define a monad that can return multiple things
• Some also define Delay or Delay and Run to handle laziness

... so computation expressions would need to be defined as quite a few different interfaces. I think the current design leaves some nice flexibility in what features of computations you can support.